This invention relates to a looper control system for controlling the looper operational angle and the tension between stands in a continuous rolling mill.
In a continuous rolling mill, as the important element for evaluating the quality of products, there are thickness, width, crown, and flatness of strips. Since the value of the tension between stands exerts a large influence on these elements, it is desirable to keep it constant as far as possible. For this reason, in the hot continuous rolling mill, such control is conducted to absorb changes in the tension value by looper mechanisms provided between respective rolling mill stands.
Hitherto, in a looper control system for a continuous rolling mill provided with loopers between stands, the tension between stands, i.e., interstand tension is controlled by allowing the looper drive motor to generate a predetermined torque, and by changing the difference between interstand speeds of the main motor, to adjust the looper operational angle. However, with this system, the length of material between stands is changed by modifying the speeds of the main motor nearby the looper. As a result, the looper operational angle control is carried out following the change in the strip length between stands. Accordingly, this leads to the drawback that the interstand tension fluctuation caused by the looper operational angle control becomes large.
Further, when a control for looper operational angle is carried out in order to reduce such a tension fluctuation, the response in the looper operational angle control must be lowered, resulting in the drawback that the control cannot cope with high frequency disturbances.
On the other hand, in order to solve the above-mentioned problems, as a looper control system to which the optimum control theory is applied, there have been proposed systems, such as, for example, disclosed in the Japanese Patent Publication No. 44129/84, the Japanese Patent Applications Laid Open Nos. 86919/83, 18213/84, 118214/84.
Japanese Patent Publication No. 44129/84 is directed to a system constructed to handle, as the state variable in the optimum control theory, looper operational angular velocity, looper operational angle, tension between stands, and difference between interstand material speeds, thus, to constitute a feedback based on only the proportional operation. However, where there is steadily a relatively large disturbance, or where the target tension changes with time, any offset is produced because there is no integral term, and results in lowered controllability. In an extreme case, a break in the strip results.
The Japanese Patent Laid Open No. 86919/83 is directed to a system constructed to add the integrating operation to the technique disclosed in the Japanese Patent Publication No. 44129/84 to thereby set a reference value by itself and remove the above-mentioned offset.
In these systems disclosed in the Japanese Patent Publication No. 44129/84 and the Japanese Patent Application Laid Open No. 86919/83, the proportional operation for deviation in connection with the reference value of the Taylor expansion used when linearly approximating the controlled system is performed. Accordingly, there remain problems to be solved, such as, for example, it is necessary to input this reference value when actually carrying out a control, working stability is poor because the proportional operational components appear to a great extent at the time of the start of control where the deviation from the reference value is large, and the like.
In the Japanese Patent Laid Open No. 118213/84, with a view to solving this problem, there is proposed a looper control system in a form to reconsider the control model from the standpoint of integral-type optimum regulator theory and perform the integral operation by integrating a deviation from the target value with respect to the variable having a control target value, and to perform the proportional operation with respect to the deviation from a lock-on value (a value at the time of start of control) subject to the proportional operation.
In addition, to Japanese Patent Application Laid Open No. 118214/84 is directed to a system to employ the looper drive motor rotational speed control in place of the looper drive motor current control unit in the configuration of Japanese Patent Application Laid Open No. 118213/84 and thereby provide an improved stability of the looper operational angle.
However, when an attempt is made to apply the above-described four proposed systems constructed from the standpoint of the optimum control of an actual rolling process, there are the problems that the control system is apt to be affected by the influence of noise of the detection signal because there are many feedback loops, that it takes much time for adjustment of the control gain, and that the control with respect to the rolling process characteristic change followed by the rolling speed change must be adjusted.
Further, the conventional looper control system only considers the rotational speed of the main motor in determining the optimum gain. However, because of roll gap correction operation by the automatic gage control (AGC), a sudden change in the tension would occur, resulting in the possibility that an extreme tension may take place. However, any effective action is not taken in the conventional system.
In addition, an effectively functioning angular range exists in the looper. That is, when a looper angle exceeds above a predetermined angle, there is the possibility that the looper may be broken. In contrast, when the looper angle is below the predetermined angle, the looper cannot entirely conduct the adjustment of the tension. Accordingly, it is necessary to operate the looper within a fixed allowable range with respect to the looper angle. However, any effective action is also not taken in this respect in the conventional system.